The present application relates to electrical power switching with smart transfer switches, and more particularly to implementation of soft-loading automatic transfer switches which employ closed-transition transfer switches.
Note that the points discussed below may reflect the hindsight gained from the disclosed inventions, and are not necessarily admitted to be prior art.
A transfer switch is an electrical switch that switches a load between two sources. Some transfer switches are manual, in that an operator effects the transfer by throwing a switch, while others are automatic, and switch when they sense one of the sources has lost or gained power.
An Automatic Transfer Switch (“ATS”) is often installed where a backup power source is located, so that the backup can provide temporary electrical power if the utility source fails. The backup power source can be, for example, a fossil fuel generator, renewable energy power source (such as solar, water, or wind power), and/or batteries.
As well as transferring the load to the backup power source, an ATS may also command a local generator to start, based on the voltage monitored on the primary supply. The transfer switch isolates the local generator from the electric utility when the generator is on and providing temporary power. The control capability of a transfer switch may be manual only, or a combination of automatic and manual. The switch transition mode (see below) of a transfer switch may be Open Transition (OT) (the usual type), or Closed Transition (CT)).
For example, in a home equipped with a backup generator and an ATS, when an electric utility outage occurs, the ATS will tell the backup generator to start. Once the ATS sees that the generator is ready to provide electric power, the ATS breaks the home's connection to the electric utility and connects the generator to the home's main electrical panel. The generator supplies power to the home's electric load, but is not connected to the electric utility lines. It is necessary to isolate the generator from the distribution system—not only to protect the generator from overload in powering loads in the house, but also for safety, as utility workers expect the lines to be dead.
When utility power returns for a minimum time, the transfer switch will transfer the house back to utility power and command the generator to turn off.
A transfer switch can be set up to provide power to critical loads only, or to entire electrical panels (or subpanels). Some transfer switches allow for load shedding or prioritization of optional circuits, such as heating and cooling equipment. More complex emergency switchgear used in large backup generator installations permits soft loading, allowing load to be smoothly transferred from the utility to the synchronized generators, and back; such installations are useful for reducing peak load demand from a utility.
An open transition transfer switch is also called a break-before-make transfer switch. A break-before-make transfer switch breaks contact with one source of power before it makes contact with another. This configuration is simple, and prevents backfeeding from an emergency generator back into the utility line. Such switches are commonly used in conventional open transition automatic transfer switches; during the split second of the power transfer, the flow of electricity is interrupted. Another common example is a manual three position circuit breaker, with utility power on one side, the generator on the other, and “off” in the middle, which requires the user to switch through the full disconnect “off” position before making the next connection.
A typical emergency system uses open transition, so there is an inherent momentary interruption of power to the load when it is transferred from one available source to another (keeping in mind that the transfer may be occurring for reasons other than a total loss of power). In most cases this outage is inconsequential, particularly if it is less than ⅙ of a second.
A closed transition transfer switch (“CTTS”) is also called a make-before-break transfer switch. There are some loads that are affected by even the slightest loss of power. There are also operational conditions where it may be desirable to transfer loads with zero interruption of power when conditions permit. For these applications, closed transition transfer switches can be provided. The switch will operate in a make-before-break mode provided both sources are acceptable and synchronized. Typical parameters determining synchronization are: voltage difference less than 5%, frequency difference less than 0.2 Hz, and maximum phase angle between the sources of 5 electrical degrees. This means that the fossil fuel engine driving the generator supplying one of the sources generally must be controlled by an isochronous governor. Batteries, and most renewables energy systems such as photovoltaic, use power inverters or converters that synchronize electronically to the grid sourcing current at the grid voltage and frequency. When disconnected from the grid, this power conversion equipment must generate the proper voltage and frequency, in a fashion similar to its operation when the grid is active.
It is generally required that the closed transition, or overlap time, be less than 100 milliseconds. If either source is not present or not acceptable (such as when normal power fails) the switch must operate in a break-before-make mode (standard open transition operation) to ensure no backfeeding occurs.
Closed transition transfer makes code-mandated monthly testing less objectionable because it eliminates the interruption to critical loads which occurs during traditional open transition transfer.
With closed transition transfer, the on-site fossil fuel engine generator set is momentarily connected in parallel with the utility source. This requires getting approval from the local utility company. Photovoltaic and battery power conversion equipment must adhere to safety standards for grid connected export of power such as UL 1741 and IEEE1574.
Typical load switching applications for which closed transition transfer is desirable include data processing and electronic loads, certain motor and transformer loads, load curtailment systems, or anywhere load interruptions of even the shortest duration are objectionable. A CTTS is not a substitute for a UPS (uninterruptible power supply); a UPS has a built-in stored energy that provides power for a prescribed period of time in the event of a power failure. A CTTS by itself simply assures there will be no momentary loss of power when the load is transferred from one live power source to another. As result CTTS are often installed with UPS systems that use fossil fuel generations, and will be increasingly used also with battery and renewable energy systems that can support loads when the utility grid is not available.
A soft-loading transfer switch (“SLTS”) makes use of a CTTS, and is commonly used to synchronize and operate onsite generation in parallel with utility power, and to transfer loads between the two sources while minimizing voltage or frequency transients.
A static transfer switch uses power semiconductors such as Silicon-controlled rectifiers (SCRs) to transfer a load between two sources. Because there are no mechanical moving parts, the transfer can be completed rapidly, perhaps within a quarter-cycle of the power frequency. Static transfer switches can be used where a reliable and independent second source of power is available and it is necessary to protect the load from even a few power frequency cycles interruption time, or from any surges or sags in the prime power source.
Conventionally, an Automatic Transfer Switch (ATS) uses electromechanical relays to transfer power. Power transfer can typically be completed within approximately 8-16 msec, or a half power cycle at a typical US power frequency of 60 Hz. As of 2012, a single phase rack mounted automatic transfer switch cost approximately $100-$150/kVA.
A Static Transfer Switch (“STS”), on the other hand, conventionally uses semiconductor switches (typically thyristors or silicon controlled rectifiers) to transfer power. Power transfer can typically be completed within about 4 msec, or approximately a quarter power cycle at a power frequency of 60 Hz. In 2012, the cost for a single phase rack mount STS was approximately $550-$700/kVA. Static transfer switches are inherently better suited to sensitive equipment than automatic transfer switches, as typically such sensitive applications require power transitions to be completed in a quarter cycle or better. Conventional static transfer switches, however, suffer from relatively high efficiency losses of 0.5-1%, and also tend to have significantly higher failure rates than automatic transfer switches. FIG. 2A shows an example of a conventional STS system.
Increased demand for consistent and reliable power, aging of electrical grids, frequent power failures, regulations in the industry and the growth of data centers and generator businesses are the major factors that have had a favorable impact on the global transfer switch market. All these factors have resulted in the transfer switch market growing extensively throughout the world. The revenue of the global transfer switch market was $624 m in 2005. The market has traditionally witnessed a growth of 4-5% per year, with the revenues increasing every year. The boom in the data centers and the IT industry has also helped in pushing the market even further. The backup generator industry has already proven to be a high value market, and this has driven the transfer switch market to even higher levels. After a minor setback with the financial crisis in late 2008, the market once again grew rapidly, and consequently, decent growth was witnessed in the transfer switch business. By 2010, the revenue of the global transfer switch market was around $800 m. The market is also expected to show significant growth in the future. It is expected that the market will reach a value of $1,521 M in 2020.
A primary application for transfer switches today is for use in data centers, but other critical load requirements are also becoming more and more prevalent. As grid connected battery systems are more widely deployed in there will be increasing demand to include transfer switches to allow these battery assets to also be used for backup power if even for a short period of time. At present, a 150 A 480V STS (115 kW) costs about $10,000, or $87/kW.
These systems weigh about 1000 lbs. Peak efficiency for these systems is approximately 99%, and more likely closer to 98% for most products.
US application US 2014-0375287 (which is hereby incorporated by reference) describes a new kind of semiconductor device, referred to herein as a “B-TRAN.” A B-TRAN is a symmetric bidirectional bipolar junction transistor, which has two separate base contact regions near the two separate emitter/collector regions. Preferably the bulk of a semiconductor die provides the base, in either direction of current flow; but since the two different base contact regions are operated separately (and differently), this type of device is referred to herein as a “double-base” transistor. B-TRANs provide a very low forward voltage drop, and hence operate very efficiently as switching devices.
The present application teaches, among other innovations, smart transfer switch circuits, using double-base bipolar transistors which have low on-state series resistance as the switching elements. In one preferred class of embodiments, these transistors are implemented as B-TRANs. Various embodiments described below permit the transfer switch to operate efficiently without requiring break-before-make operation.
In one class of embodiments, the transfer switch has its own local control circuitry, which provides a feedback output to shift the phase of one of the lines.
In one class of embodiments, the transfer switch is integrated in a single module with a PPSA (Power Packet-Switching Architecture) power converter, and both use double-base bipolar transistors.
Several specific inventive configurations are also disclosed, which provide important synergies in addition to those of the combinations just mentioned.
The present application also teaches, among other innovations, methods for automatic transfer switch operation using low-loss bidirectional switches.
The present application also teaches, among other innovations, systems which make use of the circuits, modules, and methods described herein.